In traditional automotive control systems, the low pressure servo is referenced to case pressure and the high pressure servo is referenced to the charge pressure prior to (upstream of) the automotive control orifice. In pumps without the automotive control option, the displacement control supply is traditionally common with the supply to the hydrostatic loop replenishing check valves. In units where a common supply is maintained, the common supply causes any flow demand by the hydrostatic loop to alter the input signal to the control. The varying input signal causes the control to have less than optimum, and sometimes unpredictable, performance.
In other systems, the displacement control supply is isolated from the hydrostatic loop replenishing check valves. Such systems use additional valves such as 24 to provide the isolation of the displacement control, and require additional hydraulic passageways to provide the hydraulic connections for the extra valves.